Typically, in order to enable a receiving end of an OFDM system to correctly demodulate data sent by a transmitting end, the OFDM system has to realize accurate and reliable time synchronization between the transmitting end and the receiving end. At the same time, since the OFDM system is very sensitive to the frequency offset of a carrier, the receiving end of the OFDM system also has to adopt an accurate and efficient carrier frequency estimation method, so as to precisely estimate and correct the carrier frequency offset.
At present, a signal of an OFDM system is composed of physical frames, and each physical frame generally has one synchronization frame head referred to as a preamble symbol or bootstrap, for realizing the time and frequency synchronization between a transmitting end and a receiving end. The preamble symbols is known to both the transmitting end and the receiving end, and is generally referred to as a P1 symbol. The usage of the P1 symbol or bootstrap symbol includes:
1) Enabling the receiving end to make a detection rapidly to determine whether a signal transmitted in a channel is a signal desired to be received;
2) providing a basic transmission parameter (e.g. the number of FFT points, frame type information, etc.), so that the receiving end can performing subsequent receiving processing;
3) detecting an initial carrier frequency offset and a timing error, and compensating to achieve frequency and timing synchronization; and
4) an emergency alarm or broadcast system wakeup.
A P1 symbol design based on an existing time-domain structure is proposed in existing standards such as DVB_T2 standard, which well achieves the above-mentioned functions. However, there are still some limitations on low-complexity receiving algorithms. By way of example, in the case of long and multi-path channels with 1024, 542, or 482 samples, rough timing synchronization will cause great deviation, thus leading mistake when estimating integral frequency offset of the carrier in the frequency domain. Further, in a complex frequency selective fading channel, for example in a long multi-path channel, DBPSK differential decoding method may also fail. Moreover, since the DVB_T2 time-domain structure does not include a cyclic prefix, when channel estimation needs to be conducted by utilizing the preamble symbol, the frequency-domain channel estimation performance thereof will be severely degraded.